1. Field of the Invention
The present invention relates to a process and apparatus for applying dry film solder mask material to the surface of a printed circuit board.
2. Description of the Prior Art
A solder mask is a hard, permanent layer of electrically non-conductive material which covers the surface of a printed circuit board and encapsulates the traces of the printed circuit. The solder mask is patterned to fully cover the circuitry except for portions that are intentionally left exposed for the soldering thereto of other components.
Solder masks, typically, are formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. The photoimageable layer is exposed to an actinic radiation pattern as determined by a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent or an aqueous solution which washes away either exposed or unexposed portions of the layer, depending upon whether the photoimageable layer is positive or negative acting. The portion of the layer remaining on the surface is then cured, as with heat and/or ultra violet radiation, to form a hard, permanent solder mask.
One method for applying a layer of photoimageable composition to the surface of the circuit board is to apply the composition in liquid form. After drying or partially curing, the liquid composition forms a semi-stable layer.
There are a number of advantages to applying a photoimageable layer to a circuit board in the form of a dry film rather than in liquid form. Dry films that are free of organic solvents are commercially available. The use of dry film eliminates organic solvent hazards from the work place and the need, also, for apparatus to protect the immediate work environment and the general environment from organic solvent emissions.
Typically, a dry film comprises a cover sheet of support material that is somewhat flexible but which has sufficient rigidity to provide structural support for a layer of a photoimageable composition which overlies one surface of the support sheet. The cover sheet may be formed of a polyester material such as polyethylene terephthalate (PET), such as that sold as MELINEX.RTM..
For the protection of the photoimageable layer and to enable the dry film to be rolled, it is conventional to cover the exposed surface of the photoimageable layer with a removable protective sheet of polyethylene. An example of such a dry film is sold as LAMINAR DM.RTM.by the Dynachem Division of Morton Thiokol, Inc., the assignee of the present invention.
It is the practice in the prior art to remove the protective polyethylene sheet from the photoimageable composition immediately prior to application of the dry film to the surface of the printed circuit board. Specifically, an automated apparatus peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the printed circuit board with the photoimageable layer in direct contact with the board surface. Using heat and vacuum, the photoimageable layer is immediately laminated to the surface of the board. The polyester cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from damage tending to be caused as the result of handling.
The dry film is exposed to patterned actinic radiation through the PET cover sheet. The polyester cover sheet is then removed, permitting access to the photoimageable layer by developer. Depending upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent, aqueous developer, or semi-aqueous developer, that is, a developer including both water and organic solvent. Most photoimageable composition layers require some cure subsequent to development to cause the layer to become hard and permanent and useful for serving as a solder mask. Curing may be effected with heat and/or ultra violet radiation depending upon the composition of the photoimageable layer.
Because the polyester cover sheet is relatively rigid, the photoimageable layer cannot fully conform to the irregular contours of the printed circuit board, from the flat surface of which circuitry traces project outward. As a result, the thickness of the photoimageable layer, when laminated to the board, must be slightly greater than the projection of the traces outward of the surface of the board for the photoimageable composition to overlie the outer surfaces of the traces. Thus, outer regions of photomask exist between the portions of the circuitry from which the photoimageable composition is removed during the development of the solder mask and to which a component is to be soldered. Such outer regions pose a problem particularly with respect to components which are to be surface-mounted to the printed circuit board.
Some circuit traces are narrower at the foot thereof than at the top. In such cases, small overhangs of the photoimaging layer tend to extend outwardly along each side of each trace (an effect known as "mushrooming"). With both liquid compositions and conventional dry films, air-filled voids tend to remain below such overhangs. The air trapped in such voids may produce some oxidation of the circuit traces and may lead to eventual defects in the solder mask.
An improved dry film and a process for the use thereof in the formation of a solder mask on the surface of a printed circuit board in order to eliminate the foregoing problems of both liquid and conventional dry films are disclosed and claimed in application for U.S. Pat. 4,889,790 issued Dec. 26, 1989, to Leo Roos, Frederick J. Axon and James J. Briguglio, assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference. The improved dry film includes a cover sheet formed of flexible but generally non-conforming polyester material such as polyethylene terephthalate which gives the dry film shape while allowing it to be rolled up onto a reel. On one surface of the cover sheet there is a thin layer of material, which is referred to as a top coat, and preferably, is soluble in the developer for the photoimageable material. The photoimageable composition forms a layer over the other surface of the top coat. A removable sheet of polyethylene protects the photoimageable composition layer. The material of the top coat is selected for transparency, strength and flexibility, and is further characterized in being more adherent to the photoimageable composition layer than to the cover sheet.
In the formation of a solder mask, as disclosed in the Roos et al. application, the protective, removable sheet of polyethylene is peeled away, and the exposed . surface of the photoimageable composition layer is tacked to a minor portion of the surface of the printed circuit board. The polyester cover sheet is removed, leaving the top coat as a protective covering on the layer of photoimageable composition. The layer of photoimageable composition at this time is firmly laminated to the surface of the board with the use of heat and vacuum in order to conform the photoimageable composition layer to the contours of the surface of the board.
In the application of the dry film solder mask material, it is necessary to cover the printed circuit board with a loose sheet of film before vacuum lamination. The film sheet must allow the draw-off of the air enclosed between it and the surface of the circuit board before applying mechanical pressure to conform the film to the board surface. The film outer surface is protected, during application, by the top coat. The polyester cover sheet may be removed prior to the vacuum application process to allow for good conformation of the film to the board.